Phomopsis subordinaria field survey
We found that P. subordinaria was widely spread in P. lanceolata populations in the Åland Islands as in 47.7% (124) of the sampled 260 populations; one or more infected plants were found (Figure 1A). Large populations were more commonly infected than small, and host population connectivity was positively associated with P. subordinaria infection (Table 1, Figs. 1B-C). We also found regional spatial variation in infection prevalence that was the highest in Hammarland, where 67.5% of the 37 surveyed host populations were infected, and lowest in Saltvik where 31.6% of the 19 surveyed host populations were infected (Table 1, Figs. 1A).
Within infected host populations, P. subordinaria population size and infection prevalence were typically low. In nearly half of the (48.4%) populations, we found 1-10 infected plants (Figure 1E). In 36.3% of the infected populations 11-100 infected plants were found, and in only 1.6% populations we found more than 1000 infected individuals (Figure 1E). Within populations less than 10% of the plants were infected in the majority of infected populations (87.9%), and in only 2.4% of the populations we observed more than 25% infected plants (Figure 1F). At the plant level, we found that P. subordinaria infections were highly virulent. On average 84.9% of stems were infected within infected plants (Figure 1D).
When we tested factors affecting pathogen within-population prevalence, regional districts differed in the proportion of plants infected and within-host infection load measured as proportion of infected stalks had small but significant positive effect on within-population prevalence (Table 1; Fig 1G). Neither host population connectivity nor size influenced P. subordinaria within-population prevalence (Table 1). None of the tested variables - regional district, host population connectivity and size, P. subordinaria epidemic size, or within-population prevalence – had a significant effect on within-host infection load (Table 1).